1. Field of the Invention
The present invention is directed to a self-regulating hydraulic control system employed to drive the piston of an hydraulic cylinder in a working cycle in which, during its working stroke, the piston moves against a minimal resistance during an initial or "free travel" portion of its extending stroke and then encounters a substantial resistance to further movement as, for example, in a punch press when the punch driven by the piston initially encounters the workpiece.
In such applications, of which a punch press is but one example, to minimize cycle time it is desirable to supply a high volume flow of fluid, which may be at a relatively low pressure, to the head end of the cylinder to drive the piston in rapid movement through the free travel portion of its working stroke to rapidly bring the tool or punch into contact with the work. Once the punch is engaged with the work, however, a substanial increase in pressure applied to the head end of the cylinder will be required to drive the punch through the work. This phase of the working stroke may be most efficiently accomplished, from the standpoint of the power requirement, by a pump which discharges a relatively low volume of fluid at a relatively high pressure. Where a single pump is employed to drive a piston in such a working stroke, the tonnage required during the working portion of the stroke will impose a compromise between minimizing cycle time and minimizing the horsepower requirements for the motor which drives the pump.
2. Description of the Prior Art
This problem has been recognized in the prior art and has resulted in various so called "high-low" systems which, by employing two pumps acting in conjunction with each other during the free travel portion of the piston stroke, achieve relatively rapid cycle times with a power requirement which is but a fraction of the power requirement of a single pump system. In such systems, the main pump, which will power the cylinder during the working portion of its stroke, is a relative low volume pump which can produce the desired tonnage with a relatively low power input requirement. The second pump is a relatively high volume pump which, when driven by the same motor which drives the main pump, will put out a relatively high volume of fluid at a relatively low pressure. Typically, the system is so designed that both pumps supply fluid to the cylinder during the free travel portion of the piston stroke and the output of the high volume pump is diverted from the cylinder during the working portion of the piston stroke. Such systems typically disconnect the output of the high volume pump from the working circuit by mechanically or electrically actuated valves which, in turn, rely upon pressure switches, limit switches, cams, etc., to control their actuation. These systems, in general, have proven difficult to maintain and troubleshooting is frequently complicated by the initial problem of determining whether the fault lies with the mechanical, electrical or hydraulic components of the system.
The present invention is directed to a two-pump system in which the system is self-regulating in response to its own internal operating pressures.